1. Field of Invention
The invention relates to a semi-continuous emulsion polymerization process and to a method for preparing toner particles wherein the latex is formed by emulsion polymerization with no nonionic surfactant present. The aforementioned toners are especially useful for imaging processes, especially xerographic processes, which usually require high toner transfer efficiency, such as those having a compact machine design or those that are designed to provide high quality colored images with excellent image resolution and signal-to-noise ratio, and image uniformity.
2. Description of Related Art
It is known in the art to form toners by aggregating a colorant with a latex polymer formed by batch or semi-continuous emulsion polymerization. For example, U.S. Pat. No. 5,853,943 (hereinafter "the 943 patent"), which is herein incorporated by reference, is directed to a semi-continuous emulsion polymerization process for preparing a latex by first forming a seed polymer. In particular, the 943 patent describes a process comprising:
(i) conducting a pre-reaction monomer emulsification which comprises emulsification of the polymerization reagents of monomers, chain transfer agent, a disulfonate surfactant or surfactants, and optionally, but preferably, an initiator, wherein the emulsification is accomplished at a low temperature of, for example, from about 5.degree. C. to about 40.degree. C.; PA1 (ii) preparing a seed particle latex by aqueous emulsion polymerization of a mixture comprised of (a) part of the monomer emulsion, from about 0.5 to about 50 percent by weight, and preferably from about 3 to about 25 percent by weight, of the monomer emulsion prepared in (i), and (b) a free radical initiator, from about 0.5 to about 100 percent by weight, and preferably from about 3 to about 100 percent by weight, of the total initiator used to prepare the latex polymer at a temperature of from about 35.degree. C. to about 125.degree. C., wherein the reaction of the free radical initiator and monomer produces the seed latex comprised of latex resin wherein the particles are stabilized by surfactants; PA1 (iii) heating and feed adding to the formed seed particles the remaining monomer emulsion, from about 50 to about 99.5 percent by weight, and preferably from about 75 to about 97 percent by weight, of the monomer emulsion prepared in (ii), and optionally a free radical initiator, from about 0 to about 99.5 percent by weight, and preferably from about 0 to about 97 percent by weight, of the total initiator used to prepare the latex polymer at a temperature from about 35.degree. C. to about 125.degree. C.; and PA1 (iv) retaining the above contents in the reactor at a temperature of from about 35.degree. C. to about 125.degree. C. for an effective time period to form the latex polymer, for example from about 0.5 to about 8 hours, and preferably from about 1.5 to about 6 hours, followed by cooling.
In addition, the 943 patent teaches forming the in situ seed by adding a major, for example, about 50% or more, specifically from about 50 to about 95%, of the sulfonated emulsifier/surfactant to the portion of monomers used for forming the seed polymer, which is, for example, from about 0.5 to about 50% by weight, and preferably from about 3 to about 25 percent by weight, of the total monomers used to prepare the copolymer resin.
In known emulsion polymerization processes, surfactants (that is, emulsifiers) are used to stabilize the emulsion during emulsion polymerization. The presence of good surfactants is important for stabilizing the emulsion polymerization process. Generally, the surfactants include both ionic and nonionic surfactants. However, the same surfactants that contribute advantage in the emulsion polymerization step can be detrimental to the functional properties or processing of the final toners. In particular, the presence of surfactants, particularly nonionic surfactants, can contribute to problems such as filter blinding, over-dispersed particles, persistent emulsion and/or, more importantly, undesirable final toner characteristics, such as sensitivity to relative humidity, low tribo charge, dielectric loss, aging and poor toner flow.
Current emulsion aggregation processes have a disadvantage in that tribo charge depends on environmental changes to a large extent. Tribo charge degradation is observed especially in an environment of high temperature and high humidity. The tribo charge of the emulsion aggregation toner particles at high relative humidity can generally be controlled by avoiding the presence of surfactants, particularly nonionic surfactants, on the particle surface. Another disadvantage is that the adhesive properties between the toner particles and the substrate is poor at high relative humidity owing to the presence of surfactants, particularly nonionic surfactants, on the particles.
As a result, surfactants used in emulsion aggregation emulsion polymerization processes should be removed from the particle by washing to obtain useful tribo electric properties. However, surfactants for emulsion polymerization, particularly nonionic surfactants, are known to form hydrogen-bonded complexes with carboxylic acids and are thus difficult to remove from the surface of acrylic acid-containing particles in particular. In addition, often the removal of these surfactants, particularly nonionic surfactants, from the emulsion aggregation particles is very tedious and resource consuming, since surfactant removal is an equilibrium process and requires acceleration in order to be cost-effective.
Although the 943 patent suggest that nonionic surfactants may only be optional in the process for forming a latex taught therein, nonionic surfactants are used in most of the specific embodiments taught in the 943 patent. In embodiments in which a nonionic surfactant is not used, a monomer emulsion is formed using about 33% by weight of the anionic surfactant. About 5% by weight of the monomer emulsion and an initiator solution are then added to an aqueous phase containing the remainder of the anionic surfactant (about 67% by weight) to form a seed polymer. Thereafter, the monomer emulsion is continuously fed to the aqueous phase over a period of over 4 hours and the polymerization is completed.